1. Technical Field
The present invention relates generally to a driving circuit to drive a transistor-transistor logic (TTL) level signal on to multimode receiver with terminators as differential or single ended signals. Particularly, the circuit is used to drive a reset line to a small computer system interface (SCSI) in either single ended (SE) or low-voltage differential (LVD) mode.
2. Description of Related Art
Two types of interfaces are typically present in a computer system: single ended or differential lines. The single ended transmits a bit onto a single line or wire. A positively active signal is considered xe2x80x9cactivexe2x80x9d when the voltage level is positive and xe2x80x9cinactivexe2x80x9d when the voltage level is zero or negative. On the other hand, a negatively active signal is considered xe2x80x9cactivexe2x80x9d when the voltage level is low or zero or negative and xe2x80x9cinactivexe2x80x9d when the voltage level is high or positive. When lines are configured according to differential lines, two wires or lines having voltage differentiation are used. A differential output circuit has first and second output lines.
A line driver improves output current or administers a conversion of a single ended line into a pair of differential lines or vice versa. Differential lines provide better noise immunity than a single-ended line. Differential lines are typically used when driving long line lengths in electrically noisy environments. A disadvantage of using a differential-mode is that it requires twice as many signal lines as a single-ended line. For this reason, a device may have different interfaces.
Small computer system interface (SCSI) devices have three different interface protocols: single-ended (SE); low-voltage differential (LVD); or high-voltage differential (HVD). Each of the interface lines, control and data lines, should be terminated properly at both ends to avoid reflections and to maintain the integrity of the signals. Consequently, SE lines are terminated differently than HVD lines (SCSI, SCSI-2, and SCSI-3 devices) and LVD lines (Ultra SCSI-2 devices). For further details of SCSI terminations, please refer to component UCC5628 Multimode SCSI 14 Line Terminator made by Unitrode. Consequently, a wide variety of different SCSI devices exists in the industry today.
Previous circuit design requires two separate complicated drivers, one for LVD mode termination and another for SE mode termination. A human operator must select between the drivers. Unfortunately, once selected, if the SCSI device is replaced with another, the operator must again select between the drivers according to the new device being connected.
Therefore it would be advantageous to have an improved circuit and method for reducing the complexity of drivers for LVD mode termination and SE mode termination without the driver having to detect either mode.
The present invention includes a driving circuit and method for driving signals. An input signal is received by the driving circuit on an input signal line which is connected to a bias circuit for a common voltage level. Two output lines from the driving circuit are driven to the receiver which is capable of using differential output lines or a selected single ended output line. Furthermore, the output lines may be driven to a high impedance selected by the voltage level of the input signal. The receiver of the output lines may be a SCSI device using multimode terminators which include low voltage differential and a single ended mode.
Particularly, the present invention includes a first switch which generates a voltage level coupled to a second switch that drives a signal onto the first output line, and a third switch that drives another signal onto the second output line. The first and third switches are controlled by voltage level of the input signal. The second switch is controlled by the voltage level generated by the first switch. The voltage level generated by the first switch is connected to a bias circuit for voltage reference.